Colloidal magnesium aluminum hydroxide and heterocoagulation with a clay mineral. I. Properties of colloidal magnesium aluminum hydroxide

Abstract

Magnesium aluminum hydroxide, the most important member of layered double hydroxides, was peptized by intense washing. The particle diameter, 70–130 nm, depended on the temperature of aging the parent material. The electrophoretic mobility of the particles decreased with increasing pH, from 3.7 × 10−8 m2/Vs at pH 5 to 0.5 × 10−8 m2/Vs at pH 12.3. An isoelectric point at pH∼7 was reached with the addition of 87 mmol/l NaSCN, 3 mmol/l Na2SO4 and Na2CO3, and 0.7 mmol/l Na2HPO4. The critical coagulation concentration for the 2% (w/w) dispersion was 88 mmol/l NaCl, 1.8 mmol/l Na2CO3, 1.4 mmol/l Na2SO4, and 1.2 mmol/l Na2HPO4 at pH∼7. The 2% dispersion at pH∼7 showed an almost Newtonian flow behavior. Yield values were developed after salt addition. The 2% dispersion reached a yield value of 2 Pa at 100 mmol/l NaCl, 3 Pa at 100 mmol/l Na2SO4, and 5 Pa at 100 mmol/l Na2CO3. Sodium phosphate in comparison with the other salts showed a liquefying effect. The yield value increased to 3 Pa at 1–10 mmol/l Na2HPO4 and decreased to 0.5 Pa at 100 mmol/l Na2HPO4.